The present invention relates to a method and system for signalling between a mobile station and a. network, and more particularly, to signalling between a mobile station and a network for the provision of supplementary services.
Radiotelecommunication technology has exploded in the past decade as a convenient and efficient alternative to conventional communication systems such as the telephone. Mobile telephones, in particular, have found widespread acceptance as personal communication devices due to their portability. The phrases "mobile station" and "mobile unit" as they are used herein refer not only to car-mounted units, but also to any unit which is portable, e.g., personal hand-held units.
The rapid growth in this industry has forced system designers to find ways to increase the capacity of these systems to satisfy the rapidly growing demand. Increasing capacity, however, presents numerous challenges to system designers because the frequency spectrum available for radiotelecommunications is relatively limited. Thus, designers must contend with the inherent tension between the desire to increase capacity and maintain high quality communications on the one hand, and the limited available frequency spectrum and interference considerations on the other hand.
As general demand for radiotelecommunications increases, so too does the demand for mobile system capabilities beyond those which simply establish calls. Users of mobile systems want the additional services which they have become accustomed to in fixed communication networks, for example, call forwarding, call waiting, and conference calling. Those familiar with the communication art will recognize that these are simply examples of the numerous additional services which are now available for users of fixed communication networks. These services are collectively referred to hereafter as supplementary services.
To provide supplementary services in radiocommunication networks, signalling procedures need to be established for the basic functions which comprise the supplementary services. For example, one such basic function which is used to handle several calls is called the "switch and disconnect" function where an active call is released and a held call is retrieved substantially simultaneously. These signalling procedures are combined to define a format which is commonly called the specification of the "air interface" between the mobile units and the network in a radiocommunication system. Thus, when a mobile unit requests a supplementary service, both the network and the mobile use the signalling procedures which are specified by the air interface for that particular system to provide the requested service. An example of such signalling procedures can be found in the "CCITT Recommendations Q.930-Q.932".
In many of the signalling procedures specified in the CCITT Recommendations, the mobile station plays a relatively large role. For example, in a "switch and disconnect" function set forth in the CCITT Recommendations, the mobile station sends both a signal requesting that a call be released and another signal requesting the retrieval of a held call. Similarly, in a procedure for setting up a second call to a particular mobile while a first call remains established, two signals are sent by the mobile station.
Another supplementary service which can be provided in radiocommunication networks is a mobile subscriber's request to send DTMF (Dual Tone Multi-Frequency) tones to a remote party during an active call. Systems for generating DTMF tones and the use of DTMF tones in radiocommunication systems are well known in the art and, therefore, are not further described here. For background purposes, however, U.S. Pat. No. 5,140,627 describes an exemplary DTMF signalling system which is incorporated here by reference.
In digital cellular systems, for example, DTMF tones normally can not be sent directly from a mobile station to a remote party because a speech codec provided in mobile stations for transmitting speech information between a mobile station and the network is not suited for the transfer of DTMF tones. Thus, DTMF service is typically provided by a mobile subscriber sending a Layer 3 message to a network asking that certain digits be sent to a remote party using DTMF tones.
These conventional signalling procedures are problematic in at least two respects. First, the number of signals being sent over the air interface should be reduced so that system capacity can be maximized. Second, many of the conventional signalling procedures which require the mobile station to play a relatively large role consequently require the mobile station to be relatively sophisticated and expensive.